Environ. Sci. Technol. 2002, 36, 1757-1762
Mechanisms of Arsenic Uptake from Aqueous Solution by Interaction with Goethite, Lepidocrocite, Mackinawite, and Pyrite: An X-ray Absorption Spectroscopy Study MORAG L. FARQUHAR,† JOHN M. CHARNOCK,† FRANCIS R. LIVENS,‡ AND D A V I D J . V A U G H A N * ,† Department of Earth Sciences and Williamson Research Centre for Molecular Environmental Science, and Department of Chemistry, University of Manchester, Manchester M13 9PL, U.K.
The mechanisms whereby As(III) and As(V) in aqueous solution (pH 5.5-6.5) interact with the surfaces of goethite, lepidocrocite, mackinawite, and pyrite have been investigated using As K-edge EXAFS and XANES spectroscopy. Arsenic species retain original oxidation states and occupy similar environments on the oxyhydroxide substrates, with first-shell coordination to four oxygens at 1.78 Å for As(III) and 1.69 Å for As(V). In agreement with other workers, we find that inner sphere complexes form, apparently involving bidentate (bridging) arsenate or arsenite. Interaction of As(III) and As(V) with the sulfide surfaces shows primary coordination to four oxygens (As-O: 1.69-1.76 Å) with further sulfur (∼3.1 Å) and iron (3.4-3.5 Å) shells suggesting outer sphere complexation. Arsenic species were also coprecipitated with mackinawite (pH 4.0), and these samples were further studied following oxidation. At high As(III) or As(V) concentrations, arsenate or arsenite species form, probably as sorption complexes, along with poorly crystalline arsenic sulfide (the only product at low As(V) concentrations). All oxidized samples show primary coordination to four oxygens at 1.7 Å, indicating As(V); these arsenates may show both outer sphere complexation with residual mackinawite and inner sphere complexation with new oxyhydroxides. These experiments help to clarify our understanding of As mobility in near-surface environments.
Introduction The mobility of arsenic in surface and near-surface waters is governed by a variety of factors, which include solubilities in waters of differing compositions and the interaction of solution species with the surfaces of major mineral phases. Particularly important in the latter cases are the hydrated oxides of iron (e.g., goethite, lepidocrocite), which occur very widely as colloidal materials in natural waters or as coatings on the surfaces of detrital minerals, and the iron sulfides * Corresponding author phone: (+44) 161-275-3935; fax: (+44) 161-275-3947; e-mail:
[email protected]. † Williamson Research Centre for Molecular Environmental Science. ‡ University of Manchester. 10.1021/es010216g CCC: $22.00 Published on Web 03/15/2002
2002 American Chemical Society
(mackinawite, pyrite), which play a similar role in the reducing environments just beneath the surface of many sediments in estuarine and near-shore systems. A predictive understanding of the uptake and release of arsenic by mineral surfaces requires knowledge of mechanisms at the molecular scale. As part of a larger program of investigation involving a range of mineral substrates and ions in solution, we have used X-ray absorption spectroscopy (XAS) to study these mechanisms (e.g., refs 1-3). In the case of arsenic, XAS has also been used to study speciation in solution (4). The solubility, and hence the bioavailability and toxicity, of arsenic also depends on its oxidation state, with the As(V) species dominating under oxic conditions (oxyanions such as H2AsO4- and HAsO42-) and As(III) species under reducing conditions. The As(III) form is generally more toxic than As(V) (5). Previous studies have shown that As(V) oxyanions are strongly “sorbed” to the surfaces of such iron oxides as goethite, ferrihydrite, and hematite (6-12); more recent work has also included As(III) species (13, 14). In the present study, the uptake of both As(V) and As(III) species from aqueous solution through interaction with iron oxyhydroxides and sulfides has been investigated. The specific substrates investigated have been goethite, lepidocrocite, mackinawite, and pyrite. In addition, the interaction of arsenic species with mackinawite has been studied through coprecipitation of the arsenic with FeS, and the behavior of the product of this process has been studied following reoxidation. The overall objective of these studies has been to shed further light on the mechanisms whereby arsenic in these two major oxidation states in aqueous solution interacts with the iron oxyhydroxides and sulfides.
Experimental Section In this work, powdered samples of the mineral phases were reacted with solutions containing the arsenic. The form in which the arsenic was taken up by each mineral was then investigated using X-ray absorption spectroscopy. Sample Preparation. Goethite and lepidocrocite were synthesized according to Schwertmann and Cornell (15), and the purity and homogeneity were confirmed by powder X-ray diffraction (XRD). Goethite and lepidocrocite prepared by this method typically have surface areas of 20 and 70-80 m2 g-1, respectively. Single crystals of natural pyrite were ground in a mortar and pestle and then sieved through a 32 µm sieve. The
